1. Field of the Invention
This invention relates to an improvement in a vehicle suspension system for steerable wheels.
2. Description of the Prior Art
A conventional vehicle suspension system for steerable wheels, provided with the following construction, has been generally known in the art. In a conventional vehicle suspension system, a wheel side portion and a vehicle body side portion of a motor vehicle are interconnected by a suspension arm. This same suspension arm, thus, supports the wheel swingably in the vertical direction to the vehicle body. (For example, refer to U.S. Pat. No. 4,448,441.)
As shown in FIG. 9, in this type of a vehicle suspension system, an outboard end of a suspension arm 85 is interconnected, through a ball joint 86, with a steering knuckle 84 which rotatably supports a wheel 82, while an inboard end of the suspension arm 85 is supported at a vehicle body side member 87 by two supporting portions which are longitudinally spaced apart at a fixed interval by means of a front rubber bushing 91 located substantially sideways from the steering knuckle 84 (the center of the wheel 82) and also a rear rubber bushing 92 located in the rear. One of the rubber bushings 91, 92 provided at the two supporting portions arranged in the front and rear, usually the front rubber bushing 91 located sideways from the steering knuckle 84, is made of relatively hard rubber. In other words, rigidity of the rubber bushing 91 is set higher. This is to obtain transverse rigidity of a steering device 81. On the other hand, the rear rubber bushing 92 which is diagonally spaced away from the steering knuckle 84 is made of relatively soft rubber. Thus, rigidity of the rubber bushing 92 is set lower in order to obtain the longitudinal compliance. With these arrangements, vehicle running stability and comfortableness to ride has been improved.
However, the disadvantages arise in this conventional suspension device 81 when force in the longitudinal direction acts on the wheel 82, for example, in the case where the vehicle runs over a projection on a road surface. More specifically, as shown in the broken line of FIG. 9, when force F proceeding from the front to the rear of the vehicle body acts on the wheel 82, a displacement of the rear supporting portion of the suspension arm 85 occurs not only in the rearward direction, but also in the inboard direction of the vehicle body. This is because rigidity of the rear rubber bushing 92 is set lower than the front rubber bushing 91. As a result, whole portion of the suspension arm 85 slightly rotates toward the rear with the front supporting portion as an axis. In this way, a displacement in the transverse direction also occurs in the rear end portion of the suspension arm 85 or in the rear rubber bushing 92.
To control the suspension arm 85 properly in response to the longitudinal force acting on the wheel 82, it is necessary to obtain desirable displacement characteristic in the transverse direction of the rear rubber bushing 92. Generally, torsional deformation of the rear rubber bushing 92 occurs along with the swing motion of the suspension arm 85 which is caused by the vertical displacement of the wheel 82. At the same time, the longitudinal displacement of the rear rubber bushing 92 (along with the longitudinal movement of the suspension arm 85) is needed. In this way, the rear rubber bush 92 must meet these complicated deformation requirement. Accordingly, it is rather difficult to achieve the desirable transverse displacement characteristic for the rear rubber bushing 92.
The desirable characteristics of the transverse displacement for this rear rubber bushing 92 are as follows.
(1) As mentioned earlier, when the longitudinal force acts on the wheel 82 while the vehicle is running on a rough road surface or running at high speed, the suspension arm 85 swings on the front supporting portion (the rubber bushing 91) as an axis. This leads to the swing motion of the steering knuckle 84. Accordingly, a displacement in the widthwise direction of the vehicle body occurs in a tie rod 89 which is connected with the rear end of the steering knuckle 84. In this case, the widthwise displacement of the tie rod 89 results in the generation of force to rotate a steering wheel, thus causing the so-called shimmy phenomenon. This shimmy phenomenon has a negative effect on the steering stability of the vehicle. Thus, the transverse rigidity of the rear rubber bushing 92 should desirably be increased in order to restrict this shimmy phenomenon.
(2) When the wheel runs over a projection on a road surface, the rear rubber bushing 92 is required to have extensively low transverse rigidity with softness itself. This is to ease the shock imposed on the vehicle body effectively. Meanwhile, in the region where the rubber bushing undergoes the substantial displacement in the transverse direction due to force with heavy load, the rear rubber bushing 92 is required to have high rigidity with hardness itself in order to restrict further displacement. Therefore, as for the characteristic of the rubber bushing 92 against external force when the vehicle is running on to a projection on a road, a non-linear type characteristic in accordance with the shock imposed on the vehicle body is desirable.
As mentioned above, wide variety of characteristics is required for the rear rubber bushing 92 as to the transverse elastic displacement thereof. However, it is extremely difficult to satisfy the above-mentioned transverse displacement characteristics when it comes to a rear rubber bushing which is subject simultaneously to the rotational and torsional deformation, and the longitudinal displacement.